The present invention relates generally to apparatus and methods for regulating the flow rate of gas discharged from a compressor, and more particularly to a novel bleed air control system adapted to assure a constant minimum discharge flow rate in a compressor used to power pneumatically-operated aircraft accessory system and the like.
In addition to their traditional propulsion functions, gas turbine engines are often used as accessory power units (APU's) to supply mechanical and/or pneumatic power to a wide variety of aircraft accessory devices and systems. Accessory system pneumatic power is typically provided by forcing bleed air from the APU compressor section through a main bleed duct to the accessory system's supply inlet via a branch supply duct connected to and defining a terminal portion of the main duct. In order to prevent surge of the APU compressor used to power the pneumatic accessory system, it is necessary to maintain a certain minimum flow rate through the main bleed duct.
However, the APU-supplied accessory system normally has a widely fluctuating compressed air requirement and is automatically controlled to correspondingly regulate the amount of bleed air it receives from the compressor by modulation of an accessory valve positioned in the branch supply duct.
To accommodate a decrease in accessory air demand, and maintain the compressor through flow above its surge level, a surge bleed duct is typically connected to the main bleed duct to provide an alternate outlet flow path for the compressor bleed air as flow through the branch supply duct is diminished by a closing of the accessory system valve. Flow through the surge bleed duct is regulated by modulating a surge bleed valve positioned therein.
Conventional bleed air control systems employ mechanical devices, such as diaphragm controllers, to proportionally operate the surge bleed valve in response to deviations in main duct flow rate from a desired value thereof. More specifically, as the main duct flow rate begins to deviate from a predetermined value, an error signal is generated and the control system responsively modulates the surge bleed valve to a degree directly proportional to the magnitude of the error signal.
Such conventional control of the surge bleed valve requires that the valve be initially opened at a total compressor bleed air level substantially higher than the minimum flow level (i.e., a flow level exceeding the surge level by a reasonable margin of safety) required to prevent surge of the compressor. The early surge valve opening, necessitated by the steady-state droop characteristics of proportional control which cause the surge valve operating line to be angled relative to the compressor surge line, results in a sizable amount of excess surge bleed air being dumped to atmosphere as the surge valve is moved toward its fully open position. This heretofore unavoidable excess surge bleed air causes increased APU fuel consumption, results in increased surge bleed noise, decreases total power available from the APU, and limits the maximum supply pressure available to the pneumatically-powered accessory system.